Voltage measurement instrument and method having improved automatic mode operation

ABSTRACT

Voltage measuring instruments and methods of measuring the amplitude of an input signal are disclosed. In an example method of measuring the amplitude of an input signal, in an AC measurement mode, the input signal is coupled through a capacitor, the RMS amplitude of the input signal coupled through the capacitor is determined, and a digital value corresponding to the RMS amplitude is provided. In an automatic measurement mode, the RMS amplitude of the input signal is determined without first coupling the input signal through the capacitor, and a digital value corresponding to the RMS amplitude of the input signal is provided. Further disclosed is a calibration procedure may be used to compensate for any offset of an amplifier of the voltage measuring instrument.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/633,979, filed Dec. 4, 2006. This application is incorporated byreference herein in its entirety and for all purposes.

TECHNICAL FIELD

This invention relates to test and measurement equipment, and, moreparticularly, to a circuit and method for switching a signal to analternating current (“AC”) and direct current (“DC”) voltage measurementinstrument in a manner that allows accurate voltage measurements.

BACKGROUND OF THE INVENTION

Instruments for measuring the voltage of an AC or DC signal have been inexistence for a long time, and they are in common use in a wide varietyof fields. Early voltage measuring instruments, commonly known as“volt-ohm meters,” used an analog meter having a pointer with an angularposition indicative of the amplitude of a DC voltage. Such meters couldalso measure the amplitude of AC signals by rectifying the AC signal andscaling the amplitude to correspond to the root-mean-squared (RMS″)value of the signal. Later volt-ohm meters used a digital display, andwere substantially more accurate than analog meters.

Although conventional volt-ohm meters could measure the RMS amplitude ofan AC signal, the accuracy of such measurement was based on theassumption that the AC signal had a sinusoidal waveshape. Such volt-ohmmeters could not accurately measure the RMS amplitude of anon-sinusoidal AC signal. For this reason, true RMS meters weredeveloped using a variety of techniques.

A block diagram of a conventional AC/DC voltage measuring instrument 10is shown in FIG. 1. The instrument 10 includes a pair of terminals 12,14 between which the signal to be measured is applied, generally using atest probe. The terminal 14 is coupled to ground, and the terminal 12 iscoupled to several devices. More specifically, the terminal 12 iscoupled through a resistor 18 to a comparator and logic circuit 20,through a resistor 24 and switch 26 to test node 30, and through acapacitor 32, resistor 34 and a switch 36 to the test node 30. The testnode 30 is connected to ground through a resistor 38 to form a voltagedivider with one of the resistors 24, 34.

The test node 30 is connected to an analog-to-digital (“A/D”) converter40 through a switch 42. A digital signal at the output of the A/Dconverter 40 is applied to a display 44, and/or it may be applied toother circuits or systems (not shown). The test node 30 is alsoconnected to an input of an amplifier 46. An output of the amplifier 46is coupled through a capacitor 48 to an RMS circuit 50, which providesan accurate measurement of RMS amplitude of a signal applied to itsinput. An output of the RMS circuit 50 is coupled through a switch 54 tothe A/D converter 40.

The voltage measuring instrument 10 has three operating modes, namely aDC measurement mode, an AC measurement mode, and an automaticmeasurement mode. These modes are selected by a control unit 60selectively closing the switches 26, 36, 42, 54. The control unit 60 is,in turn, controlled by either a user selection device 62 or an outputfrom the comparator and logic circuit 20, as explained in greater detailbelow. The control unit 60 is also connected to the display 44 so thatit can show the mode that is currently in use. In the DC measurementmode or the AC measurement mode, the control unit 60 causes the display44 to display whatever mode is selected through the user selectiondevice 62. However, in the automatic measurement mode, the control unit60 is controlled by the comparator and logic circuit 20 to cause thedisplay 44 to indicate “Automatic” and to also display “AC” if thecomparator and logic circuit 20 detects zero crossings and to otherwisedisplay “Automatic” and “DC.”

When the amplitude of a DC voltage is to be measured, the DC measurementmode is selected through the user selection device 62. The selectiondevice 62 then causes the control unit 60 to close the switches 26 and42 while the switches 36 and 54 remain open. Closing of the switch 26connects the input terminal 12 to the test node 30 so that the resistor24 forms a voltage divider with the resistor 38. The amplitude of thevoltage at the test node 30 is thus proportional to the amplitude of thesignal applied between the input terminals 12, 14. The test node 30 isconnected by the closed switch 42 directly to the A/D converter 40. TheA/D then outputs a digital signal indicative of the amplitude of thesignal applied between the terminals 12, 14, and the display 44 providesthe user with an indication of the amplitude of the DC voltage appliedbetween the terminals 12, 14.

When the amplitude of an AC voltage is to be measured, the ACmeasurement mode is selected, again through the user selection device62. The selection device 62 then causes the control unit 60 to close theswitches 36 and 54 while the switches 26 and 42 remain open. Closing ofthe switch 36 connects the input terminal 12 to the test node 30 throughthe capacitor 32 so that only an AC signal is coupled through theresistor 34, which forms a voltage divider with the resistor 38. Theamplitude of the voltage at the test node 30 is thus proportional to theamplitude of the AC signal applied between the input terminals 12, 14.The amplitude of this AC signal at the test node 30 is boosted by theamplifier 46 and coupled through the capacitor 48 to the input of theRMS circuit 50. Although the capacitor 32 passes only AC signals, use ofthe capacitor 48 is desirable to eliminate offsets that are typicallygenerated by the amplifier 46. In the AC measurement mode, the output ofthe RMS circuit 50 is coupled through the closed switch 54 to the A/Dconverter 40. The A/D converter 40 thus outputs a digital signalindicative of the RMS amplitude of the AC signal applied between theterminals 12, 14. The RMS amplitude of this signal is then displayed bythe display 44.

In the automatic measurement mode, the voltage measuring instrument 10switches between the DC measurement mode and the AC measurement modebased on the nature of the signal applied between the terminals 12, 14.This function is accomplished by the comparator and logic circuit 20detecting zero crossings of the signal to be measured. If the circuit 20detects zero crossings of the signal applied between the terminals 12,14, it assumes the signal is an AC signal. It therefore sends acorresponding signal to the control unit 60, which closes the switches36, 54 to place the instrument 10 in the AC measurement mode, asexplained above. If the circuit 20 does not detect zero crossings, itassumes the signal applied between the terminals 12, 14 is a DC signal.The circuit 20 therefore sends a corresponding signal to the controlunit 60, which closes the switches 26, 42 to place the instrument 10 inthe DC measurement mode, as also explained above.

The voltage measuring instrument 10 shown in FIG. 1 works well in manyapplications, particularly where the signal to be measured is a pure DCsignal or a pure AC signal. However, it does not provide accurateresults where the signal to be measured is a DC signal with an ACcomponent or an AC signal having a DC offset. If the AC component issufficiently large in relation to the DC offset, the comparator andlogic circuit 20 will detect zero crossings and therefore switch theinstrument 10 to the AC measurement mode. However, the capacitors 32, 48will block the DC component so that only the AC component will bemeasured. The RMS circuit 50 will therefore produce a digital outputthat is indicative of the RMS amplitude of only the AC component. Yetthe true RMS amplitude is affected by the DC component as well as the ACcomponent.

Another problem develops if the AC component is relatively small inrelation to the DC offset. In such case, the comparator and logiccircuit 20 will not detect zero crossings and therefore switch theinstrument 10 to the DC measurement mode. The instrument 10 will thenprovide spurious measurements of the DC amplitude of the signal, whichwill vary depending upon the sample point used by the A/D converter 40.These inconsistent measurements may indicate to the user that the DCvoltage is continuously changing when, in fact, it is constant.

There is therefore a need for voltage measuring instrument and methodthat can provide accurate measurements of the voltage of signals in anAutomatic measurement mode, including DC signals with an AC component orAC signals having a DC offset.

SUMMARY OF THE INVENTION

A voltage measurement instrument and method measures the amplitude of aninput signal in either a DC measurement mode, and AC measurement mode oran automatic measurement mode. In the DC measurement mode, the amplitudeof the input signal is sampled by an analog-to-digital converter, whichgenerates a digital value corresponding to the amplitude of the sample.In the AC measurement mode, the input signal is coupled through acapacitor to an RMS circuit, which determines the RMS amplitude of theinput signal after being coupled through the capacitor. A digital valuecorresponding to the RMS amplitude is then generated. In the automaticmeasurement mode, the input signal is coupled directly to the RMScircuit without first coupling the input signal through a capacitor. Inthe automatic mode, the input signal is preferably coupled to the RMScircuit through an amplifier. The voltage measurement instrumentperforms a calibration procedure to calculate a correction factor tocompensate for any offset generated by the amplifier. This correctionfactor is used in the automatic mode to display a corrected RMSamplitude value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one example of a conventional AC/DC voltagemeasuring instrument.

FIG. 2 is a block diagram of an AC/DC voltage measuring instrumentaccording to one example of the invention.

FIG. 3 is a flow chart showing one example of a calibration procedurethat may be used to calibrate any offset in an amplifier used in thevoltage measuring instrument of FIG. 2.

FIG. 4 is a block diagram of an AC/DC voltage measuring instrumentaccording to another example of the invention.

DETAILED DESCRIPTION

A voltage measurement instrument 100 according to one example of theinvention is shown in FIG. 2. The instrument 100 has some similaritiesto the voltage measurement instrument 10 shown in FIG. 1, including theresistor 24 and switch 26 coupled between the terminal 12 and the testnode 30 used to measure the amplitude of DC signals, and the capacitor32, resistor 34 and switch 36 coupled between the terminal 12 and thetest node 30 used to measure the amplitude of AC signals. The voltagemeasuring instrument 100 also includes the switch 42, which is closed toconnect the test node 30 to the A/D converter 40 for measuring theamplitude of DC signals, and the amplifier 46, RMS circuit 50 and theswitch 54, which is closed for measuring the amplitude of AC signals.Finally, the instrument 100 also includes a control unit 104 and theuser selection device 62. However, the control unit 104 has beenprovided with a different reference number because it differs instructure and function from the structure and function of the controlunit 60 used in the instrument 10.

The voltage measurement instrument 100 of FIG. 2 differs from theinstrument 10 of FIG. 1 in several important respects. First, theinstrument 100 includes a low pass filter 110 in the path used tomeasure the amplitude of DC signals. The low pass filter 110 filters outany AC component in the signal to be measured so that the amplitudedisplay does not fluctuate with different samples taken by the A/Dconverter 40. The instrument 100 thus provides a consistent and accuratemeasurement of the amplitude of DC signals. Additionally, for reasonsthat will also be explained below, the output of the A/D converter 40 isconnected to the control unit 104.

In the AC measurement mode, the voltage measuring instrument 100operates in essentially the same manner as the instrument 10 asexplained below, except that it does not include the capacitor 48 usedby the instrument 10 to eliminate any offset in the amplifier 46.Instead, as explained in greater detail below, the control unit 104performs a calibration procedure to eliminate any offset in theamplifier 46.

The most significant difference between the voltage measuring instrument100 and the prior art instrument 10 is its operation in the automaticmeasurement mode. When the automatic measurement mode is selected usingthe user selection device 62, the control unit 104 closes the switches26, 54 and opens the switches 36, 42. Furthermore, the switches 26, 54remain closed and the switches 36, 42 remain open regardless of whetheror not the comparator and logic circuit 20 detects zero crossings. Thesole function of the comparator and logic circuit 20 is therefore tocontrol the display 44 in the same manner it controls the display 44 inthe automatic measurement mode in the prior art instrument 10, asexplained above. Unlike the comparator and logic circuit 20 in theconventional instrument 10, the comparator and logic circuit 20 in theinstrument 100 does not cause the control unit 104 to control any of theswitches 26, 54, 36, 42 in the automatic mode.

By coupling the RMS circuit 50 to the terminal 12 without anyintervening capacitor or other high pass filter, the RMS circuit is ableto provide an accurate measurement of RMS amplitude regardless of thenature of the signal. If, for example, the signal is a DC signal with alarge AC component, the RMS circuit will provide a true indication ofthe RMS value of the signal, which will take into account both the DCcomponent and the AC component of the signal.

As mentioned above, the instrument 100 is able to dispense with the useof a capacitor to eliminate offsets of the amplifier 46 because thecontrol unit 104 performs a calibration procedure. As shown in FIG. 3,this calibration procedure 120 is entered at 122 and the control unit104 then opens switches 26 and 36. Resistor 38 then brings node 30 tozero volts. The control unit 104 then examines the output of the A/Dconverter 40. If the A/D converter 40 provides an RMS amplitudeindication of other than zero, the control unit 104 determines at step128 the offset of the amplifier 46 corresponding to this RMS amplitude.The control unit 104 then calculates a correction factor at step 130that will be used for subsequent measurements in the AC measurement modeand the automatic measurement mode. Finally, the calibration procedureexits at step 134. Although one example of a calibration procedure isshown in FIG. 3, it will be understood that other calibration proceduresmay be used to compensate for any offset of the amplifier 46 and RMSconverted 50. Regardless of what calibration procedure is used, the useof a calibration procedure to compensate for offset of the amplifier 46allows the RMS circuit 50 to be connected through a DC circuit path inthe AC measurement mode and in the automatic measurement mode regardlessof the nature of the signals applied between the terminals 12, 14. As aresult, the voltage measuring instrument 100 is able to provide moreaccurate measurements when the amplitude of a signal has both AC and DCcomponents.

Another example of a voltage measuring instrument 140 is shown in FIG.4. The voltage measuring instrument 140 includes all of the samecomponents that are used in the voltage measuring instrument 100 of FIG.2 operating in essentially the same manner. The instrument 140 differsfrom the instrument 100 by connecting the comparator and logic circuit20 to the terminal 12 through a resistor 144 rather than connecting itdirectly to the test node 30 as in the instrument 100.

Although the present invention has been described with reference to thedisclosed embodiments, persons skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. Such modifications are well within the skillof those ordinarily skilled in the art. Accordingly, the invention isnot limited except as by the appended claims.

1. A method of measuring the amplitude of an input signal in a voltagemeasuring instrument having a plurality of measurement modes, the methodcomprising: in an AC measurement mode, coupling the input signal througha capacitor, determining the RMS amplitude of the input signal coupledthrough the capacitor, and providing a digital value corresponding tothe RMS amplitude; and in an automatic measurement mode, determining theRMS amplitude of the input signal without first coupling the inputsignal through the capacitor, and providing a digital valuecorresponding to the RMS amplitude of the input signal.
 2. The method ofclaim 1, further comprising amplifying the input signal in the automaticmeasurement mode before determining the RMS amplitude of the inputsignal.
 3. The method of claim 2, further comprising performing acalibration procedure in the automatic measurement mode to compensatefor any offset generated during the act of amplifying the input signal.4. The method of claim 3 wherein the calibration procedure comprises:making the input signal have a predetermined voltage; examining theamplified input signal to determine if the amplitude of the amplifiedinput signal corresponds to the predetermined voltage; if the amplitudeof the amplified input signal does not correspond to the predeterminedvoltage, calculating a correction factor; and using the correctionfactor in determining the RMS amplitude of the input signal.
 5. Themethod of claim 1, further comprising in a DC measurement mode ofmeasuring the amplitude of the input signal, sampling the amplitude ofthe input signal and converting the sample to a digital valuecorresponding to the amplitude of the sample.
 6. The method of claim 5,further comprising displaying the digital value corresponding to theamplitude of the sample in the DC measurement mode, and displaying thedigital value corresponding to the RMS amplitude in the AC measurementmode and the automatic mode.
 7. The method of claim 5, furthercomprising low-pass filtering the input signal in the DC measurementmode before sampling the amplitude of the input signal.